The present application relates to retinal prostheses and more particularly to improved electrodes for retinal stimulation.
It has been know since the 1700s that nerves carry their signals throughout the body by electricity. More recently, we have learned that we can partially control those signals by applying an electrical signal directly to, or in the proximity of, a nerve ending. One of the most difficult forms of nerve stimulation is the creation of artificial sight by electrically stimulating the retina.
U.S. Pat. No. 5,109,844 (xe2x80x9cDe Juanxe2x80x9d) and U.S. Pat. No. 5,935,155 (xe2x80x9cHumuyanxe2x80x9d) disclose systems for the electrical stimulation of the retina by a retinal electrode array held against the retina. DeJuan discloses an epiretinal electrode array. Humuyan discloses a system for capturing a video image, transferring the image wirelessly into a living body and applying the image to a retinal electrode array.
The retina is structured with its light sensitive cells (rods and cones) farthest from its surface. The light sensitive cells convert light to electricity, process the electrical signal, and pass the electrical signal back toward the surface, first through bipolar cells, then through ganglion cells, and finally through nerve fibers. The nerve fibers relay the signal across the surface of the retina to the optic nerve. Simply applying an electrical signal through surface electrodes will provide the greatest stimulus to the cells closest to the surface. This will stimulate primarily nerve fibers and ganglion cells. The nerve fibers have a resistive coat which somewhat limits the signal received directly by the nerve fibers, allowing part of the signal to reach the ganglion cells. Since nerve fibers carry signal across the surface of the retina, stimulating nerve fibers can create a percept of light in a different location than intended. Stimulating between two epiretinal electrodes may cause percepts at both the anode and cathode. Much of a signal applied the retinal surface travels along the retinal surface from one electrode to the other, stimulating no cells at all.
Most retinal diseases, primarily macular degeneration and retinal pigmentosa, affect only the light receptive cells. In this case, bipolar, ganglion, and nerve fiber cells can function normally. Since a visual image is processed in the bipolar and ganglion cells, it is most advantageous to stimulate bipolar cells.
Some have addressed this problem by lifting the retina and placing an array of stimulating electrodes under the retina, and closer to the bipolar cells. The retina is a delicate organ. Implanting a subretinal electrode array is complex and dangerous surgery. Further, a subretinal electrode primarily stimulates the defective light sensitive cells and, as with epiretinal electrodes, much of the electrical signal applied to the back surface of a retina travels along that surface between electrodes. A subretinal electrode is disclosed in U.S. patent application Ser. No. 09/515,373, filed Feb. 29, 2000, entitled Method and Apparatus for Color Sight Restoration.
A new, more efficient, electrode configuration is needed to promote deep stimulation of the retina, and the bipolar cells within.
The present invention is an improved retinal electrode array with a remote return electrode outside of the eye. An array of stimulating electrodes is placed on the retinal surface (epiretinally) or under the retina (subretinally) and a large return electrode is placed outside of the sclera and distant from the array of stimulating electrodes. The remote return electrode promotes deeper stimulation of retinal tissue.